If I do this serr, how do I tell Reaper to load FROM the samples that are now on my SSD (OS drive) vs. keep on streaming them from the sample drive?

Thanks!

Keep the same organization for your system/os/apps/library/etc on the SSD. Keep the samples in the same folder structure.

But only keep the samples you want to use here!

Now keep a folder on your data drive for the EXTRA samples.
Make sidebar shortcuts to both in the Finder (Explorer and whatever sidebar shortcuts are called in Windows) and you can quickly drag and drop back and forth if you want to add or remove from your "working" sample folder.

Reaper will of course be directed to your "working" sample folder on the SSD by default and will know nothing of the extra sample folder on your data drive.

Further though, you can also change default paths in Reaper or redirect the sampler plugin (likely with some setup script) when it works that way.